In the art of bleaching, the target is to provide pulp with high and stable brightness levels. However, brightness reversion of pulp occurs to varying extent during storage and transportation to the paper mill depending on e.g. temperature, humidity, wood species, cooking conditions and bleaching sequence. Also, dewatering and drying conditions of pulp subsequent to the final bleaching stage are of importance for brightness stability to the bleached pulp. When market pulp is to be transported from its place of manufacture to a paper mill, it should of economical and quality reasons be substantially dry. It is known that the operation of pulp drying machines can affect the brightness to varying degrees depending on the stability of the pulp and the drying conditions. For example, if the pulp is dried at high temperature and then baled, thermal brightness reversion continues during storage due to the high temperature and the humidity existing within the bale.
“The Bleaching of Pulp”, Third Edition, Tappi press, edited by Singh, R. P, p. 378, 388-390, 1979, discloses that yellowing of pulps is faster the higher the temperature and the higher the humidity. It is further disclosed that color reversion is higher the higher the pH of the water from which a pulp sheet is made. It is thus conventional in the art to carry out drying at fairly low pH no matter which bleaching sequence has been performed on the pulp to be dried.
In the paper “Variables affecting the thermal yellowing of TCF-bleached birch Kraft pulps”, Granström, A, Nordic Pulp and Paper Research Journal Vol. 16, no. 1/2001, various variables such as hexenuronic acid content, dry content, temperature, time and pH influencing the process of brightness reversion are discussed. This article shows varying degrees of brightness reversion depending on these variables, inter alia at pH values ranging from 3 to 6. Hexenuronic acid groups as mentioned in Granström are formed e.g. during alkaline pulping when 4-O-methyl-D-glucuronic acid groups of xylan are converted. The presence of hexenuronic acid groups has previously been suspected to be at least partly responsible for brightness reversion of pulps. Attempts to remove hexenuronic acid groups have therefore been made, e.g. by means of acid hydrolysis prior to bleaching or by selecting a bleaching sequence degrading hexenuronic acid groups. All these attempts, however, lead to a more complicated pulp production system and in most cases these attempts are associated with additional investments.
The present invention provides a method for improving the brightness stability of bleached pulps which have been subjected to alkaline pulping, particularly those which contain a certain degree of hexenuronic acid groups.